How do I read a Psychrometric Chart

Relative Humidity can be calculated with a wet bulb thermometer and a dry bulb thermometer, using a psychrometric chart as below.  Wet-bulb temperature is measured using a thermometer, with the glass bulb or sensor tip wrapped in a wick, which is kept wet. The evaporation of water from the wet wick has a cooling effect on the thermometer and the rate of evaporation from the wet-bulb thermometer depends on the humidity of the air. Therefore, less humidity means more cooling, which means a lower wet bulb temperature, and a bigger difference between wet and dry bulb temperatures.

The difference in the temperatures indicated by the two thermometers gives a measure of atmospheric humidity. This chart allows humidity to be calculated from wet and dry bulb readings. (The dry bulb temperature is another name for air temperature). It also explains common atmospheric behaviour that we see daily. Well break the chart down for ease of explanation:

The vertical lines represent dry bulb temperatures.


The diagonal lines represent wet bulb temperatures.


Lines of constant humidity are also shown on the chart


Read the relative humidity by finding where the wet bulb temperature and the dry bulb temperature readings intersect. The nearest constant humidity line is the relative humidity for the given wet and dry bulb temperatures. In the exampleto the left(20°C dry and 10°C wet) gives 25% relative humidity.

You can also read the wet bulb temperature from the chart, when you know the air temperature and relative humidity, using the same principle of finding the intersection of the relevant lines.